Gamma-Ray Flash Came from Star Being Eaten by Massive
Black Hole
The story is reprinted (with editorial adaptations by
ScienceDaily staff) from materials provided by
University of California - Berkeley.
ScienceDaily
June 16, 2011
http://www.sciencedaily.com/releases/2011/06/110616142709.htm
A bright flash of gamma rays observed March 28 by the
Swift satellite may have been the death rattle of a star
falling into a massive black hole and being ripped
apart, according to a team of astronomers led by the
University of California, Berkeley.
When the Swift Gamma Burst Mission spacecraft first
detected the flash within the constellation Draco,
astronomers thought it was a gamma-ray burst from a
collapsing star. On March 31, however, UC Berkeley's
Joshua Bloom sent out an email circular suggesting that
it wasn't a typical gamma-ray burst at all, but a high-
energy jet produced as a star about the size of our sun
was shredded by a black hole a million times more
massive.
Careful analysis of the Swift data and subsequent
observations by the Hubble Space Telescope and the
Chandra X-ray Observatory confirmed Bloom's initial
insight. The details are published online on June 16 in
Science Express, a rapid publication arm of the journal
Science.
"This is truly different from any explosive event we
have seen before," Bloom said.
What made this gamma-ray flare, called Sw 1644+57, stand
out from a typical burst were its long duration and the
fact that it appeared to come from the center of a
galaxy nearly 4 billion light years away. Since most, if
not all, galaxies are thought to contain a massive black
hole at the center, a long-duration burst could
conceivably come from the relatively slow tidal
disruption of an infalling star, the astronomers said.
"This burst produced a tremendous amount of energy over
a fairly long period of time, and the event is still
going on more than two and a half months later," said
Bloom, an associate professor of astronomy at UC
Berkeley. "That's because as the black hole rips the
star apart, the mass swirls around like water going down
a drain, and this swirling process releases a lot of
energy."
Bloom and his colleagues propose in their Science
Express paper that some 10 percent of the infalling
star's mass is turned into energy and irradiated as X-
rays from the swirling accretion disk or as X-rays and
higher energy gamma rays from a relativistic jet that
punches out along the rotation axis. Earth just happened
to be in the eye of the gamma-ray beam.
Bloom draws an analogy with a quasar, which is a distant
galaxy that emits bright, high-energy light because of
the massive black hole at its center gobbling up stars
and sending out a jet of X-rays along its rotation axis.
Observed from an angle, these bright emissions are
called active galactic nuclei, but when observed down
the axis of the jet, they're referred to as blazars.
"We argue that this must be jetted material and we're
looking down the barrel," he said. "Jetting is a common
phenomenon when you have accretion disks, and black
holes actually prefer to make jets."
Looking back at previous observations of this region of
the cosmos, Bloom and his team could find no evidence of
X-ray or gamma-ray emissions, leading them to conclude
that this is a "one-off event," Bloom said.
"Here, you have a black hole sitting quiescently, not
gobbling up matter, and all of a sudden something sets
it off," Bloom said. "This could happen in our own
galaxy, where a black hole sits at the center living in
quiescence, and occasionally burbles or hiccups as it
swallows a little bit of gas. From a distance, it would
appear dormant, until a star randomly wanders too close
and is shredded."
Probable tidal disruptions of a star by a massive black
hole have previously been seen at X-ray, ultraviolet and
optical wavelengths, but never before at gamma-ray
energies. Such random events, especially looking down
the barrel of a jet, are incredibly rare, "probably once
in 100 million years in any given galaxy," said Bloom.
"I would be surprised if we saw another one of these
anywhere in the sky in the next decade."
The astronomers suspect that the gamma-ray emissions
began March 24 or 25 in the uncatalogued galaxy at a
redshift of 0.3534, putting it at a distance of about
3.8 billion light years. Bloom and his colleagues
estimate that the emissions will fade over the next
year.
"We think this event was detected around the time it was
as bright as it will ever be, and if it's really a star
being ripped apart by a massive black hole, we predict
that it will never happen again in this galaxy," he
said.
Bloom's colleagues include UC Berkeley theoretical
physicist Elliot Quataert, who models the production of
jets from accretion disks, and UC Berkeley astronomers
S. Bradley Cenko, Daniel A. Perley, Nathaniel R. Butler,
Linda E. Strubbe, Antonino Cucchiara, Geoffrey C. Bower
and Adam N. Morgan; Dimitrios Giannios and Brian D.
Metzger of Princeton University; Andrew J. Levan of the
University of Warwick, Coventry, United Kingdom; Nial R.
Tanvir, Paul T. O' Brien, Andrew R. King and Sergei
Nayakshin of the University of Leicester in the U.K.;
Fabio De Colle, Enrico Ramirez-Ruiz and James Guillochon
of UC Santa Cruz; William H. Lee of the Universidad
Nacional Autonoma de México in Mexico City; Andrew S.
Fruchter of the Space Telescope Science Institute in
Baltimore, Md.; and Alexander J. van der Horst of the
Universities Space Research Association in Huntsville,
Ala.
Levan is first author of the companion Science Express
paper, and leader of the Chandra and Hubble Space
Telescope observation team.
Bloom and his laboratory are supported by grants from
NASA and the National Science Foundation.
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